Mullard M8161 - For cold winter days or insurance claims

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Joined: May 21st, '16, 05:19

Mullard M8161 - For cold winter days or insurance claims

Post by fr00tbat »

Hi. I'll start with my questions for those who don't want to wade through my blah blah.

1. How alarmed should I be by the marked increase in heat produced by Mullard M8161 (EF92) in a MK III v4?
2. Should a different set of power tubes be used with EF92 series tubes over the stock 6Н6П?
3. Would a gain setting of 4 with 600 Ohm DT 880's increase the load and heat produced when using EF92s?

First time posting, so I'll just start by saying thanks to the Little Dot crew for making a truly brilliant little bit of kit. I've been going back and listening to songs I used to love and the Mk III v4, stock tubes, and a pair of DT880s make them all sound like new. I'm amazed by tiny details I never noticed before and digital and vinyl both sound awesome. I just hope I don't go deaf from having headphones on all the time.

Right, so the other day I tried a set of Mullard M8161 (EF92) tall tubes I bought on eBay. I did remember to change the jumpers :mrgreen:. I thought they sounded really good right out of the box, but good god they got hot quickly! Definitely not a set to leave to burn in while out doing the shopping unless you're interested in shopping for a new house. There was a bit of a smell as well, so of course I was worried that something was wrong. The amp was still chugging along without as if nothing was wrong and still sounded awesome at all volumes, so I switched it off and set a fan blowing on it to cool down. I had it on for about four hours with me listening 90% of the time while working on a paper. I have run the amp on the stock 5654s for at least that, and though it does get warm it's not so hot I'm afraid to pick it up with bare hands.

Later on I put the jumpers back and reinstalled the stock 5654s. The amp still works like a champ as if nothing happened. A relief and testament to build quality.

Since the Mullards were my first set of EF92s I got curious about why they would get so hot. I was also not sure if the heat was coming from them, the power tubes (stock 6Н6П), the transformer, or the hidden bits and pieces inside the amp. Or all of the above. I have read on Head-Fi that EF92s run at 200mA while EF95s run at 175mA. Would 25mA really create that much more heat? I don't have an electronics background, so it's all magical to me. And is there a better set of power tubes to use with EF92s that are more efficient?

I do have the gain set to 4 with the 600 Ohm DT880s. Could that contribute to the amp getting so hot?

I'm going to try researching this a bit more when I have time, but I just wanted to pop in and see if anyone else had noticed this and if it's something I need to worry about. I did a search through the forum (of course), and I saw David said in other threads that these amps do get hot and EF92s get hotter, but he didn't really say it was a problem other than keeping the on time sensible. Or is it just how this series is and I just need to keep the in-use time down to prevent the amp from overheating?

Thanks and I'll update if I find anything else out...


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Joined: Mar 14th, '06, 01:25

Re: Mullard M8161 - For cold winter days or insurance claims

Post by DavidZheZhe »

Hello Justin,

The M8161 (and EF92 tubes in general) will indeed run hotter due to the higher current. The amp will run fine at the higher temperature however we do recommend keeping your Little Dot in a well ventilated area. 4x gain is on the lower side and won't add to the heat output significantly. Typically all Class-A amps will run hot to start with and the EF92s just runs a bit hotter than most other tubes.

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